Cardiac insufficiency, characterized inter alia by a reduction in the cardiac output, is a common, well-known and well-documented heart malfunction. It develops as a result of congenital defects or as an end-effect of many diseases. Cardiac output, i.e., the output of the heart per unit time, is the product of stroke volume and heart rate. Hence, variations in cardiac output can be produced by chances in cardiac rate or stroke volume. The stroke volume can be influenced, for example, by changing the strength of cardiac contraction, by changing the length of the cardiac muscle fibers, and by changing contractility of cardiac muscle independent of fiber length. The heart rate and rhythm influence the cardiac output both directly and indirectly, since changes in the rate and rhythm also affect myocardial contractility.
The human body normally regulates the cardiac output in response to body needs by changing the heart rate, as during physical exercise, and/or by adapting the stroke volume. Under pathological conditions, however, some of the normal regulatory mechanisms may be damaged. For example, heart tissue damaged due to myocardial infarct typically cannot sustain normal pumping function, leading to a reduction in stroke volume, and hence of cardiac output. The body may react to such a reduction by increasing the heart rate, thus imposing long term strain on the heart muscles, leading in more severe cases to heart failure. There is thus a need for devices and treatments that can regulate the cardiac output, so as to compensate for the deficiencies in the normal regulation mechanisms.
In response to this need, modern cardiology has developed means to control various parameters associated with the heart's operation. Pharmaceuticals, for example, may be used to influence the conduction velocity, excitability, contractility and duration of the refractory period of the heart tissue. These pharmaceuticals are used to treat arrhythmia, enhance cardiac output and prevent fibrillation. Pharmaceuticals are generally limited in effectiveness in that they affect both healthy and diseased segments of the heart, usually, with a relatively low precision. They frequently also have unwanted side-effects.
A special kind of control can be achieved using implantable electronic devices, which provide excitatory electrical stimulation to the heart to control directly the heart rate and/or rhythm. For example, a pacemaker, an electronic devices which is typically implanted in the heart to support the heart's electrical excitation system or to bypass a blocked portion of the conduction system. Another type of cardiac electronic device is a defibrillator, which senses fibrillation in the heart and applies a high voltage impulse to "reset" the heart. While electronic pacemakers can control the heart rate, however, they are limited in their capacity to enhance cardiac output, and they are known to reduce stroke volume in at least some instances. Defibrillators are useful in treating arrhythmia when it occurs (although they are painful to the patient and traumatic to the heart), but they provide no long-term amelioration of cardiac insufficiency.
Thus, none of the treatments known in the art allow effective, long-term regulation of cardiac output. The electromechanical properties of the heart, as well as methods known in the art for influencing these properties, are more fully described in the "Background of the Invention" section of PCT patent application PCT/IL97/00012, which is assigned to the assignee of the present patent application and is incorporated herein by reference.